Gulf of Tonkin incident
| combatant2 = | commander1 = George Stephen Morrison | commander2 = Le Duy Khoai . | strength1 = Sea: 1 aircraft carrier, 1 destroyer Air: 4 aircraft . | strength2 = 3 torpedo boats | casualties1 = 1 destroyer slightly damaged, 1 aircraft slightly damaged . | casualties2 = 1 torpedo boat severely damaged, 2 torpedo boats moderately damaged, 4 killed, 6 wounded . | campaignbox = }} The Gulf of Tonkin incident, or the USS Maddox incident, are the names given to two separate confrontations, one actual and one false, involving North Vietnam and the United States in the waters of the Gulf of Tonkin. On August 2, 1964, the destroyer , while performing a signals intelligence patrol as part of DESOTO operations, engaged three North Vietnamese Navy torpedo boats of the 135th Torpedo Squadron. A sea battle resulted, in which the Maddox expended over two hundred and eighty 3-inch and 5-inch shells, and in which four USN F-8 Crusader jet fighter bombers strafed the torpedo boats. One US aircraft was damaged, one 14.5 mm round hit the destroyer, three North Vietnamese torpedo boats were damaged, and four North Vietnamese sailors were killed and six were wounded; there were no U.S. casualties. . The second Tonkin Gulf incident was originally claimed by the U.S. National Security Agency to have occurred on August 4, 1964, as another sea battle, but instead may have involved "Tonkin Ghosts" . (false radar images) and not actual NVN torpedo boat attacks. The outcome of these two incidents was the passage by Congress of the Gulf of Tonkin Resolution, which granted President Lyndon B. Johnson the authority to assist any Southeast Asian country whose government was considered to be jeopardized by "communist aggression". The resolution served as Johnson's legal justification for deploying U.S. conventional forces and the commencement of open warfare against North Vietnam. In 2005, an internal National Security Agency historical study was declassified; it concludedRobert J. Hanyok, "Skunks, Bogies, Silent Hounds, and the Flying Fish: The Gulf of Tonkin Mystery, 2-4 August 1964", Cryptologic Quarterly, Winter 2000/Spring 2001 Edition, Vol. 19, No. 4 / Vol. 20, No. 1. that the Maddox had engaged the North Vietnamese Navy on August 2, but that there were no North Vietnamese Naval vessels present during the incident of August 4. The report stated regarding August 2: and regarding August 4: Background Although the United States attended the Geneva Conference (1954), which was intended to end hostilities between France and the Vietnamese at the end of the First Indochina War, it refused to sign the Geneva Accords (1954). The Accords mandated, among other measures, a temporary ceasefire line, intended to separate Vietnamese and French forces, and elections to determine the future political fate of the Vietnamese within two years. It also forbade the political interference of other countries in the area, the creation of new governments without the stipulated elections, and foreign military presence. By 1961, President Ngo Dinh Diem faced significant discontent amongst some quarters of the southern population, including some Buddhists who were opposed to the rule of Diem's Catholic supporters. After suppressing Vietminh political cadres who were legally campaigning between 1955 and 1959 for the promised elections, Diem faced a growing communist-led uprising that intensified by 1961, headed by the National Front for the Liberation of South Vietnam (NLF or, derogatively, Viet Cong). . The US had begun providing direct support to the South Vietnamese government in the form of military and financial aid and military advisers, the number of which grew from 600 in 1961 to 16,000 by the end of John F. Kennedy's presidency in 1963. The Gulf of Tonkin Incident occurred during the first year of the Johnson administration. While Kennedy had originally supported the policy of sending military advisers to Vietnam, he had begun to alter his thinking due to what he perceived to be the ineptitude of the Saigon government and its inability and unwillingness to make needed reforms (which led to a US-supported coup which resulted in the death of Diem). Shortly before his assassination in November 1963, Kennedy had begun a limited recall of US forces. Johnson's views were likewise complex, but he had supported military escalation in Vietnam as a means to challenge what he perceived as the expansionist policies of the Soviet Union. The Cold War policy of containment was to be applied to prevent the fall of Southeast Asia to communism under the precepts of the domino theory. After Kennedy's assassination, Johnson ordered in more US forces to support the Saigon government, beginning a protracted United States presence in Southeast Asia. 's explanation of the Gulf of Tonkin incident]] A highly classified program of covert actions against North Vietnam known as Operation Plan 34-Alpha, in conjunction with the DESOTO operations, had begun under the Central Intelligence Agency (CIA) in 1961. In 1964 the program was transferred to the US Defense Department and conducted by the Military Assistance Command, Vietnam Studies and Observations Group (SOG)Joint Chiefs of Staff, Military Assistance Command, Vietnam Studies and Observations Group, Documentation Study (July 1970), Annex F, Appendix x. For the maritime portion of the covert operation, ''Tjeld''-class fast patrol boats had been purchased quietly from Norway and sent to South Vietnam. Although the crews of the boats were South Vietnamese naval personnel, approval for each mission conducted under the plan came directly from Admiral U.S. Grant Sharp, Jr., CINCPAC in Honolulu, who received his orders from the White House.MACSOG Documentation Study, Appendix C, p. 14. After the coastal attacks began, Hanoi lodged a complaint with the International Control Commission (ICC), which had been established in 1954 to oversee the terms of the Geneva Accords, but the US denied any involvement. Four years later, US Secretary of Defense Robert S. McNamara admitted to Congress that the US ships had in fact been cooperating in the South Vietnamese attacks against North Vietnam. Maddox, although aware of the operations, was not directly involved. What was (and is) generally not considered by US politicians at the time (and by later historians) were the other actions taken under Operations Plan 34-Alpha just prior to the incident. The night before the launching of the actions against North Vietnamese facilities on Hòn Mê and Hòn Ngư islands, the SOG had launched a covert long-term agent team into North Vietnam, which was promptly captured. That night (for the second evening in a row) two flights of CIA-sponsored Laotian fighter-bombers (piloted by Thai mercenaries) attacked border outposts well within southwestern North Vietnam. The Hanoi government (unlike the US government, which had to give permission at the highest levels for the conduct of these missions) probably assumed that they were all a coordinated effort to escalate military actions against North Vietnam. . The incident Daniel Ellsberg, who was on duty in the Pentagon the night of August 4 receiving messages from the ship, reported that the ship was on a secret electronic warfare support measures mission (codenamed DESOTO) near Northern Vietnamese territorial waters.Polmar, Norman "The U. S. Navy Electronic Warfare (Part 1)" United States Naval Institute Proceedings October 1979 p.138 On July 31, 1964, had begun its intelligence collection mission in the gulf. Captain George Stephen Morrison was in command of local American forces from his flagship . The Maddox was under orders not to approach closer than eight miles (13 km) from the North's coast and four miles (6 km) from Hon Nieu island.Pentagon Papers When the SOG commando raid was being carried out against Hon Nieu, the ship was away from the attacked area. First attack On August 2 Maddox radioed it was under attack from three North Vietnamese Navy P-4 torpedo boats away from the North Vietnamese coast in international waters. Maddox stated she had evaded a torpedo attack and opened fire with its five-inch (127 mm) guns, forcing the torpedo boats away. Four USN F-8 Crusader jets launched from the aircraft carrier then attacked the retiring P-4s, claiming one was sunk and one heavily damaged. Maddox, suffering very minor damage from a single 14.5-millimeter machine gun bullet, retired to South Vietnamese waters where she was joined by the destroyer . The North Vietnamese claimed that Maddox was hit by one torpedo, and one of the American aircraft had been shot down. This account, however, has come into sharp dispute with an internal NSA historical studyRobert J. Hanyok, "Skunks, Bogies, Silent Hounds, and the Flying Fish: The Gulf of Tonkin Mystery, 2-4 August 1964", Cryptologic Quarterly, Winter 2000/Spring 2001 Edition, Vol. 19, No. 4 / Vol. 20, No. 1. which stated on page 17: The Maddox when confronted, was approaching Hòn Mê Island, three to four miles (6 km) inside the twelve-mile (19 km) limit claimed by North Vietnam. This territorial limit was unrecognized by the United States. After the skirmish, President Johnson ordered the Maddox and Turner Joy to stage daylight runs into North Vietnamese waters, testing the twelve-mile (19 km) limit and North Vietnamese resolve. These runs into North Vietnamese territorial waters coincided with South Vietnamese coastal raids and were interpreted as coordinated operations by the North, which officially acknowledged the engagements of 2 August 1964.OAH Magazine of History, fall 1992. The Gulf of Tonkin by Erich Martel Others, such as Admiral Sharp, maintained that U.S. actions did not provoke the August 2 action. He claimed that North Vietnamese radar had tracked Maddox along the coast, and was thus aware that the destroyer had not actually attacked North Vietnam and that Hanoi (or the local commander) had ordered its craft to engage Maddox anyway. North Vietnamese General Phùng Thế Tài later claimed that the Maddox had been tracked since July 31 and that it had attacked fishing boats on August 2, forcing North Vietnamese Navy to "fight back."“Interview with Phung The Tai, 1981.” 02/19/1981. WGBH Media Library & Archives. Retrieved 9 November 2010. Sharp also noted that orders given to Maddox to stay eight miles (13 km) off the North Vietnamese coast put the ship in international waters, as North Vietnam claimed only a five-mile (8 km) nautical limit as its territory (or off of its off-shore islands). In addition, many nations had previously carried out similar missions all over the world, and the had earlier conducted an intelligence-gathering mission in similar circumstances without incident.Admiral U.S. Grant Sharp, Strategy for Defeat — Vietnam in Retrospect (San Rafael, CA: Presidio Press, 1978) P. 42 Second alleged attack On August 4, another DESOTO patrol off the North Vietnamese coast was launched by Maddox and the Turner Joy, in order to "show the flag" after the first incident. This time their orders indicated that the ships were to close to no more than from the coast of North Vietnam. During an evening and early morning of rough weather and heavy seas, the destroyers received radar, sonar, and radio signals that they believed signaled another attack by the North Vietnamese navy. For some two hours the ships fired on radar targets and maneuvered vigorously amid electronic and visual reports of enemies. Despite the Navy’s claim that two attacking torpedo boats had been sunk, there was no wreckage, bodies of dead North Vietnamese sailors, or other physical evidence present at the scene of the alleged engagement.http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB132/essary.htm, Prados, John. The White House Tapes: Eavesdropping on the President A Book-and-CD Set. New York: New, 2003. George Washington University. Web. 25 October 2009. At 1:27am Washington time, Herrick sent a cable in which he acknowledged the attack may not have happened and that there may actually have been no Vietnamese craft in the area: "Review of action makes many reported contacts and torpedoes fired appear doubtful. Freak weather effects on radar and overeager sonarmen may have accounted for many reports. No actual visual sightings by Maddox. Suggest complete evaluation before any further action taken". . One hour later, Herrick sent another cable, stating, "Entire action leaves many doubts except for apparent ambush at beginning. Suggest thorough reconnaissance in daylight by aircraft." . In response to requests for confirmation, at around 1600 Washington time, Herrick cabled, "Details of action present a confusing picture although certain that the original ambush was bona fide." At 1800 Washington time (0500 in the Gulf of Tonkin), Herrick cabled yet again, this time stating, "the first boat to close the Maddox probably launched a torpedo at the Maddox which was heard but not seen. All subsequent Maddox torpedo reports are doubtful in that it is suspected that sonarman was hearing the ship's own propeller beat" [sic]. Within thirty minutes of the 4 August incident, President Johnson had decided on retaliatory attacks. That same day he used the 'hot line' to Moscow, and assured the Soviets he had no intent in opening a broader war in Vietnam. Early on the 5 August Johnson publicly ordered retaliatory measures stating, "The determination of all Americans to carry out our full commitment to the people and to the government of South Vietnam will be redoubled by this outrage." One hour and forty minutes after his speech, US aircraft reached North Vietnamese targets. On the 5 August at 10:40am these planes flying from US aircraft carriers, bombed four torpedo boat bases, and an oil-storage facility in Vinh.The World Today, Vol. 26, No. 5 (May, 1970), pp. 209-217, Adam Roberts United States response President Johnson's speech to the American people Shortly before midnight on August 4, President Johnson made a speech by radio in which he described an attack by North Vietnamese vessels on two U.S. Navy warships, USS Maddox and and requested authority to undertake a military response. . Johnson's speech repeated the theme that "dramatized Hanoi/Ho Chi Minh as the aggressor and which put the U.S. into a more acceptable defensive posture." Johnson also referred to the attacks as having taken place "on the high seas," suggesting that they had occurred in international waters.[http://www.presidentialrhetoric.com/historicspeeches/johnson_lyndon/gulfoftonkin.html Text of Lyndon Johnson's speech, Gulf of Tonkin Incident.] He emphasized commitment to both the American people, and the South Vietnamese government. He also reminded Americans that there was no desire for war. "A close scrutiny of Johnson's public statements...reveals no mention of preparations for overt warfare and no indication of the nature and extent of covert land and air measures that already were operational." Johnson's statements were short to "minimize the U.S. role in the conflict; a clear inconsistency existed between Johnson's actions and his public discourse." Reaction from Congress While President Johnson’s final resolution was being drafted, Senator Wayne Morse attempted to hold a fundraiser to raise awareness about possible faulty records of the incident involving the USS Maddox. Morse supposedly received a call from an informant who has remained anonymous urging Morse to investigate official logbooks of the Maddox. These logs were not available before President Johnson’s resolution was presented to Congress. After urging Congress that they should be wary of President Johnson’s coming attempt to convince Congress of his resolution, Morse failed to gain enough cooperation and support from his colleagues to mount any sort of movement to stop it. Immediately after the resolution was read and presented to Congress, Morse began to fight it. He contended in speeches to Congress that the actions taken by the United States were actions outside of the constitution and were “acts of war rather than acts of defense." Morse’s efforts were not immediately met with support, largely because he revealed no sources and was working with very limited information. It was not until after the United States became more involved in the war that his claim began to gain support throughout the United States government. The controversial Morse was defeated when he ran for re-election in 1968.Myers, Clay. Oregon Blue Book. Salem, Oregon: Office of the Secretary of State, 1970. Distortion of the event Evidence was still being sought on the night of August 4 when Johnson gave his address to the American public on the incident. Messages recorded that day indicate that neither President Johnson nor McNamara was certain of an attack. Various news sources, including Time, Life and Newsweek, ran articles throughout August on the Tonkin Gulf incident. The Emperor's New Clothes. Web. 26 October 2009. Time reported: "Through the darkness, from the West and south…intruders boldly sped…at least six of them… they opened fire on the destroyers with automatic weapons, this time from as close as 2,000 yards." Time stated that there was "no doubt in Sharp’s mind that the U.S. would now have to answer this attack", and that there was no debate or confusion within the administration regarding the incident. The use of the set of incidents as a pretext for escalation of U.S. involvement follows the issuance of public threats against North Vietnam, as well as calls from American politicians in favor of escalating the war. . On May 4, 1964, William Bundy called for the U.S. to "drive the Communists out of South Vietnam", even if that meant attacking both North Vietnam and Communist China. Even so, the Johnson administration in the second half of 1964 focused on convincing the American public that there was no chance of war between North Vietnam and the U.S. North Vietnamese General Giap suggested that the DESOTO patrol had been sent into the Gulf to provoke North Vietnam into giving an excuse for escalation of the war. Various government officials and men aboard the Maddox have suggested similar theories. American politicians and strategists had been planning provocative actions against North Vietnam for some time. George Ball told a British journalist after the war that "at that time…many people…were looking for any excuse to initiate bombing". Provocative action against North Vietnam was considered after the August, 1964 incidents John McNaughton suggested in September 1964 that the U.S. prepare to take actions to provoke a North Vietnamese military reaction, including plans to use DESOTO patrols North. William Bundy’s paper dated September 8, 1964 suggested more DESOTO patrols as well. Consequences US Defense Secretary Robert McNamara failed to inform US President Lyndon B. Johnson that the U.S. naval task group commander in the Tonkin Gulf, Captain John J. Herrick, had changed his mind about the alleged North Vietnamese torpedo attack on U.S. warships he had reported earlier that day. By early afternoon of 4 August, Washington time, Herrick had reported to the Commander in Chief Pacific in Honolulu that "freak weather effects" on the ship’s radar had made such an attack questionable. In fact, Herrick was now saying, in a message sent at 1:27 pm Washington time, that no North Vietnamese patrol boats had actually been sighted. Herrick now proposed a "complete evaluation before any further action taken." McNamara later testified that he had read the message after his return to the Pentagon that afternoon. But he did not immediately call Johnson to tell him that the whole premise of his decision at lunch to approve McNamara’s recommendation for retaliatory air strikes against North Vietnam was now highly questionable. Had Johnson been accurately informed about the Herrick message, he might have demanded fuller information before proceeding with a broadening of the war. Johnson had fended off proposals from McNamara and other advisers for a policy of bombing the North on four separate occasions since becoming President. President Johnson, who was up for election that year, ordered retaliatory air strikes and went on national television on August 4. Although Maddox had been involved in providing intelligence support for South Vietnamese attacks at Hòn Mê and Hòn Ngư, Johnson denied, in his testimony before Congress, that the U.S. Navy had supported South Vietnamese military operations in the Gulf. He thus characterized the attack as "unprovoked" since the ship had been in international waters. As a result of his testimony, on August 7, Congress passed a joint resolution (H.J. RES 1145), titled the Southeast Asia Resolution, which granted President Johnson the authority to conduct military operations in Southeast Asia without the benefit of a declaration of war. The Resolution gave President Johnson approval "to take all necessary steps, including the use of armed force, to assist any member or protocol state of the Southeast Asia Collective Defense Treaty requesting assistance in defense of its freedom." Later statements about the incident In 1965, President Johnson commented privately: "For all I know, our Navy was shooting at whales out there." In 1981, Captain Herrick and journalist Robert Scheer re-examined Herrick's ship's log and determined that the first torpedo report from August 4, which Herrick had maintained had occurred—the "apparent ambush"—was in fact unfounded. Although information obtained well after the fact supported Captain Herrick's statements about the inaccuracy of the later torpedo reports as well as the 1981 Herrick/Scheer conclusion about the inaccuracy of the first, indicating that there was no North Vietnamese attack that night, at the time U.S. authorities and all of the Maddox crew stated that they were convinced that an attack had taken place. As a result, planes from the carriers and were sent to hit North Vietnamese torpedo boat bases and fuel facilities during Operation Pierce Arrow. Squadron commander James Stockdale was one of the U.S. pilots flying overhead during the second alleged attack. Stockdale wrote in his 1984 book Love and War: "I had the best seat in the house to watch that event, and our destroyers were just shooting at phantom targets—there were no PT boats there... There was nothing there but black water and American fire power." Stockdale at one point recounts seeing pointing her guns at the Maddox. Stockdale said his superiors ordered him to keep quiet about this. After he was captured, this knowledge became a heavy burden. He later said he was concerned that his captors would eventually force him to reveal what he knew about the second incident. In 1995, retired Vietnamese Defense Minister Vo Nguyen Giap, meeting with former Secretary of Defense McNamara, denied that Vietnamese gunboats had attacked American destroyers on August 4, while admitting to the attack on August 2."McNamara asks Giap: What happened in Tonkin Gulf?". (November 9, 1995). Associated Press A taped conversation of a meeting several weeks after passage of the Gulf of Tonkin Resolution was released in 2001, revealing that McNamara expressed doubts to President Johnson that the attack had even occurred. In the Fall of 1999, retired senior CIA engineering executive S. Eugene Poteat wrote that he was asked in early August 1964 to determine if the radar operator's report showed a real torpedo boat attack or an imagined one. He asked for further details on time, weather and surface conditions. No further details were forthcoming. In the end he concluded that there were no torpedo boats on the night in question, and that the White House was interested only in confirmation of an attack, not that there was no such attack. In the 2003 documentary The Fog of War: Eleven Lessons from the Life of Robert S. McNamara, The former Secretary of Defense Robert S. McNamara admitted that the Aug 4 attack never happened. NSA report In October, 2005 the New York Times reported that Robert J. Hanyok, a historian for the U.S. National Security Agency, had concluded that the NSA deliberately distorted the intelligence reports that it had passed on to policy-makers regarding the August 4, 1964 incident. He concluded that the motive was not political but was probably to cover up honest intelligence errors. Hanyok's conclusions were initially published within the NSA in the Winter 2000/Spring 2001 Edition of Cryptologic Quarterly, about five years before they were revealed in the Times article. According to intelligence officials, the view of government historians that the report should become public was rebuffed by policymakers concerned that comparisons might be made to intelligence used to justify the Iraq War (Operation Iraqi Freedom) that commenced in 2003."Robert J. Hanyok: His [[United States National Security Council|NSC] study on Tonkin Gulf Deception]". (October 31, 2005). New York Times. Reviewing the NSA's archives, Mr. Hanyok concluded that the NSA had initially misinterpreted North Vietnamese intercepts, believing there was an attack on August 4. Midlevel NSA officials almost immediately discovered the error, he concluded, but covered it up by altering documents, so as to make it appear the second attack had happened. On November 30, 2005, the NSA released the first installment of previously classified information regarding the Gulf of Tonkin incident, including a moderately sanitized version of Mr. Hanyok's article. The Hanyok article stated that intelligence information was presented to the Johnson administration "in such a manner as to preclude responsible decision makers in the Johnson administration from having the complete and objective narrative of events." Instead, "only information that supported the claim that the communists had attacked the two destroyers was given to Johnson administration officials." With regard to why this happened, Hanyok wrote: Hanyok included his study of Tonkin Gulf as one chapter of an overall history of the involvement of NSA, and American signals intelligence (SIGINT), in the Indochina Wars. A moderately sanitized version of the overall historySpartans in Darkness: American SIGINT and the Indochina War, 1945-1975, Center for Cryptologic History, National Security Agency, 2002 was released in January 2008 by the National Security Agency and published by the Federation of American Scientists.Report reveals Vietnam War hoaxes, faked attacks See also *Gulf of Tonkin Resolution *''The Fog of War'' *Operation Pierce Arrow *Reichstag Fire Decree Notes References * * * * * External links * The Gulf of Tonkin Incident, 40 Years Later; Flawed Intelligence and the Decision for War in Vietnam — National Security Archive at George Washington University * The Gulf of Tonkin Resolution and the Escalation of the Vietnam War — EDSITEment lesson from the Natioanl Endowment for the Humanities * NSA declassified documents released on 11/30/05 * Vietnam Study, Casting Doubts, Remains Secret * Tonkin Gulf reports cooked? Historian's research finds intelligence errors covered up * LBJ's address to the nation, August 5, 1964 * Pentagon PapersLink * Cronkite: Gulf of Tonkin's Phantom Attack * US Navy Historical Site showing charts and photos of the incident * Analysis Casts Doubt on Vietnam War Claims — Calvin Woodward, Associated Press, December 1, 2005. "A spy-agency analysis released Thursday contends a second attack on U.S. ships in the Gulf of Tonkin never happened, casting further doubt on the leading rationale for escalation of the Vietnam War." * Robert J. Hanyok, "Skunks, Bogies, Silent Hounds, and the Flying Fish: The Gulf of Tonkin Mystery, 2–4 August 1964" Cryptologic Quarterly, Winter 2000/Spring 2001 Edition, Vol. 19, No. 4 / Vol. 20, No. 1. * Tonkin Gulf Intelligence "Skewed" According to Official History and Intercepts — National Security Archive at George Washington University * Ronnie E. Ford "New Light on Gulf of Tonkin" * Original Document: Tonkin Gulf Resolution Category:Vietnam War Category:History of the United States (1964–1980) Category:20th-century military history of the United States Category:History of cryptography Category:International maritime incidents Category:Maritime incidents in Vietnam Category:National Security Agency Category:Signals intelligence Category:Combat incidents Category:1964 in Vietnam Category:Conflicts in 1964 Category:Naval battles of the Vietnam War Category:Naval battles of the Vietnam War involving the United States Category:Battles and operations of the Vietnam War in 1964 Category:Gulf of Tonkin Category:1964 in military history bg:Тонкински инцидент ca:Incident del Golf de Tonquín cs:Incident v Tonkinském zálivu da:Tonkin-episoden de:Tonkin-Zwischenfall es:USS Maddox (DD-731)#Incidente del Golfo de Tonkín fr:Incidents du golfe du Tonkin ko:통킹 만 사건 it:Incidente del Golfo del Tonchino hu:Tonkini incidens nl:Tonkin-incident ja:トンキン湾事件 no:Tonkin-episoden pl:Incydent w Zatoce Tonkińskiej pt:Incidente do Golfo de Tonkin ru:Инцидент в Тонкинском заливе sk:Tonkinský incident sr:Инцидент у Тонкиншком заливу fi:Tonkininlahden välikohtaus tr:Tonkin Körfezi Olayı uk:Тонкінський інцидент vi:Sự kiện Vịnh Bắc Bộ zh:北部湾事件